1. Technical Field
The present invention relates generally to liquid crystal (LC) phase-retarders realized by a liquid crystal film structure having one or more phase retardation regions formed therein, with each region having an optical axis and a phase retardation specified by the direction and depth of orientation of liquid crystal molecules along the surface of the liquid crystal film structure, and more particularly relates to linear cholesteric liquid crystal (CLC) polarizers realized by forming one or more phase retardation regions within CLC film structures.
2. Brief Description of the State of the Art
In the contemporary period, there is a great need to modulate the spatial intensity of light in liquid crystal display (LCD) panels, optical computing systems, holographic information storage and retrieval systems and the like. In nearly all such optical applications, linear polarizers are required to carry out this light intensity modulation function. In one form or another, a pair of linear polarizers are rotated relative to each other to produce a filter structure having a particular light transmittivity.
For example, in LCD panels, light produced by a backlighting structure (e.g. fluorescent tubes) is intensity modulated over pixel-sized regions by polarization-controlled pixel elements realized over the surface of the LCD panel. In particular, each pixel element is typically realized by interposing liquid crystal material between a pair of optically transparent electrodes which are connected to a computer-controlled voltage source response to digital image data sets corresponding to color or gray-scale images to be visually displayed. This liquid crystal structure is then disposed between a pair of linear polarizing filters. When a voltage level is impressed across the electrodes of each pixel element, an electric field is produced to thereacross, causing the polarization direction of light transmitted from the first linear polarizing filter to rotate an amount proportional to the electric field strength and the light exiting from the second linear polarizing filter to be reduced in intensity. Accordingly, by simply controlling the electric field strength across the linear polarizing filter elements at each pixel element in an LCD panel, it is possible to control the intensity of light transmitted therefrom and thus form images at the surface of the LCD panel.
Presently, there exist two different types of polarizers, namely: dichroic (i.e. sheet) linear polarizers which operative upon an absorptive mechanism; and cholesteric liquid crystal (CLC) naturally-circular polarizers which operate upon a non-absorptive mechanism. It will be useful to briefly describe each of these linear polarizing structures below.
Dichroic linear polarizers were first invented by Edwin Land back in the early 1940""s. This type of linear polarizing structure is based on a mechanism which absorbs and converts into heat the component of incident light along a first polarization direction P1, while transmitting without energy absorption the component of incident light along the desired orthogonal polarization direction, P2. Typically, such inefficient conversion of photonic energy results in the production of heat over the surface of the polarizer, causing undesired changes in the polarization characteristics of the polarizer, and at high intensities of incident light, the destruction of the polarizer. Also, by virtue of the inherent inefficiency of this type of polarizer, the use of dichroic polarizers in the construction of prior art LCD panels causes an inherent reduction in brightness by a factor of at least 50 percent. Yet, notwithstanding such to shortcomings and drawbacks, the fact that dichroic linear polarizer can be produced in large surface areas and at low cost and weight, has lead to widespread use in LCDs and thus the proliferation of laptop computers.
In recent times, super broadband and narrow-band CLC-based linear polarizers have been developed for use in various optical applications. Exemplary structures can be found in the following publications: International Application Serial No. PCT/US96/17464 entitled xe2x80x9cSuper Broad-band Polarizing Reflective Materialxe2x80x9d, by Sadeg M. Faris, et al., published under International Publication Number WO 97/16762 on May 9, 1997; and EPO Application No. 94200026.6 entitled xe2x80x9cCholesteric Polarizer and Manufacture Thereofxe2x80x9d. Both of these publications are incorporated herein by reference as if set forth in their entirety.
One of the principal advantages of both narrow and broadband cholesteric polarizers alike is that such polarizers make it possible to very efficiently convert unpolarized light into circularly polarized light without the undesired absorption of photonic energy, characteristic of dichroic polarizers. The reason for this advantage is that narrow-band and broadband CLC films alike exhibit polarization and wavelength dependent reflection properties by virtue of the helical ordering of the CLC molecules in such films.
In particular, narrow-band CLC films having left handed helical ordering selectively reflect the left hand circularly polarized (LHCP) component of incident light having wavelengths within the narrow band (e.g. 10-50) nanometers), while transmitting through the polarizer the right hand circularly polarized (RHCP) component of incident light over that narrow-band. Narrow-band CLC films having right handed helical ordering selectively reflect the RHCP component of incident light having wavelengths within the narrow band while transmitting through the polarizer the LHCP component of incident light over that narrow-band.
Similarly, broadband CLC films having left handed helical ordering selectively reflect the LHCP component of incident light having wavelengths within the band of 400-800 nanometers, while transmitting through the polarizer the right hand circularly polarized (RCP) component of incident light over that narrow-band. Broadband CLC films having right handed helical ordering selectively reflect the RHCP component of incident light having wavelengths within the band of 400-800 nanometers, while transmitting through the polarizer the LHCP component of incident light over that narrow-band.
In their native form, the prior art CLC-based (i.e. cholesteric) polarizers are limited to optical applications where either left-handed or right-handed circularly polarized light is required, as described above. However, in many applications such as LCD panels, linear polarized light is required, as described above. Consequently, circularly polarized light produced from CLC circularly polarized film must be converted to linearly polarized light. In prior art CLC polarizers, polarization conversion is carried out by passing the circularly polarized light through a quarter wave phase retardation film. Preferably, this is achieved by laminating the quarter-wave retardation film onto the surface of the CLC-based circular polarizing film. While this composite structure can be produce linearly polarizing light without absorbing photonic energy and producing heat, characteristic of dichroic linear polarizers, such a composite linear polarizer suffers from a number of shortcomings and drawbacks.
In particular, when laminating the quarter-wave retardation film onto the CLC film, the designer is constrained to use only non-birefringent adhesives, to maximize transmissions of the desired polarization state and to avoid reduction in polarizer extinction ratio. To ensure optimal light transmission through the composite structure, it is necessary to match the refractive indices of the CLC film and the quarter-wave retardation film. Also, to avoid delamination of these films, it is necessary to thermally match the coefficients of thermal expansion thereof, which is not easily achieved. Collectively, these conditions and constraints render the manufacture of such non-absorbing linear polarizers very difficult, and greatly increase the cost of manufacture of such linear polarizing structures that utilizes a quarter-wave retardation.
Thus, there is a great need in the art for a non-absorbing linear polarizer and a method and apparatus for making the same, while avoiding the shortcomings and drawbacks associated with prior art CLC-based linear polarizing structures which require the lamination of quarter-wave retardation film material to a circularly polarizing CLC film structure.
Accordingly, a primary object of the present invention is to provide a liquid crystal film structure in which one or more birefringent phase-retardation regions are realized along the surface thereof, each having an optical axis specified by the direction of ordering of the liquid crystal molecules therein.
Another object of the present invention is to provide a liquid crystal linear polarizer, in which a birefringent phase-retardation region is realized along the surface of a cholesteric liquid crystal (CLC) film structure without laminating a quarter-wave phase retardater thereto, as required by prior art liquid crystal polarizing structures.
Another object of the present invention is to provide a liquid crystal film structure with one or more birefringent phase-retardation regions formed on the first and/or second surfaces thereof, wherein the direction of the optical axis of each surface-based phase-retardation region can be arbitrarily selected in order to any particular application at hand.
Another object of the present invention is to provide such a liquid crystal film structure, wherein the amount of phase retardation imparted to light passing through each birefringent phase-retardation region is determined by the surface depth of orientation of the liquid crystal molecules within the phase retardation region, and the optical axis of the phase retardation region is determined by the direction of molecular orientation effected therein.
Another object of the present invention is to provide a cholesteric liquid crystal (CLC) film structure with a birefringent phase-retardation region formed on one surface thereof, so that a cholesteric linear polarizer with a single characteristic polarization direction is realized in a single CLC film structure.
Another object of the present invention is to provide a cholesteric liquid crystal film structure with one or more birefringent phase-retardation regions formed on the first and/or second primary surfaces thereof, so that a cholesteric linear polarizer with a plurality of spatially-defined characteristic polarization directions is realized in a single CLC film structure.
Another object of the present invention is to provide a micropolarization panel realized on a single sheet of CLC film, in which the left pixel regions thereof have a first linear polarization state (i.e. linear polarization direction) coincident with a first birefringent phase retardation region realized by orienting the liquid crystal molecules at a first surface depth and along first reorientation direction to form a first plurality of linear polarizing elements, whereas the right pixel regions thereof have a second linear polarization state (i.e. linear polarization direction) coincident with a second birefringent phase retardation region realized by orienting the liquid crystal molecules at a second surface depth and long second reorientation direction to form a second plurality of linear polarizing elements orthogonal to the first plurality of linear polarizing elements.
Another object of the present invention is to provide a single CLC film structure with a birefringent phase-retardation region formed in the surface thereof, wherein the amount of phase retardation imparted to light passing therethrough is determined by the surface depth of orientation of the liquid crystal molecules in the CLC film structure, and the optical axis of the phase retardation region is determined by the direction of molecular orientation effected in the surface of the CLC film.
A further object of the present invention is to provide a method and apparatus for producing such phase retardation regions in a sheet of CLC film, wherein reorientation of the cholesterically ordered molecules along the surface of the film is achieved by mechanically rubbing or burnishing the CLC film surface along the desired direction of molecular reorientation, and at a surface pressure sufficient to achieve the depth of molecular reorientation required to achieved the desired to phase retardation over the burnished region.
A further object of the present invention is to provide a novel tool for performing molecular reorientation of the cholesterically ordered molecules along selected regions of the surface of CLC film material.
A further object of the present invention is to provide a method and apparatus for producing such birefringent phase retardation regions in a sheet of soft (i.e. unpolymerized) CLC film, wherein orientation of the nematically ordered molecules along the surface of the film is achieved during fabrication using UV light which aligns the liquid crystal molecules along the desired direction of molecular orientation, and at a molecular depth sufficient to achieve the desired phase retardation and optical axis direction over the treated region.
A further object of the present invention is to provide a novel system for reorientating liquid crystal molecules along selected regions of the surface of CLC film, using UV light.
Another object of the present invention is to provide a CLC-based linear polarizer for use in constructing the pixel elements of LCD panels having high-brightness characteristics.
Another object of the present invention is to provide a CLC film structure with one or more birefringent phase-retardation regions realized along the first and second principal surfaces thereof, by orienting the liquid crystal molecules along a particular orientation direction with the region, wherein each phase retardation region has an optical axis aligned along the direction of molecular orientation.
These and other objects of the present invention will become apparent hereinafter and in the Claims to Invention.